The present invention relates to an apparatus and method for use in conjunction with graphics equipment incorporating controlled multiple marking devices to create displays, and more particularly, such apparatus and method for use in determining the position of the marking devices along axes of movement.
Digital plotters and similar graphics equipment frequently utilize pencils, pens or other marking devices to develop a plot, graph, or similar graphics. Typically, in an XY plotter, the paper, chart or other medium to be marked is moved through the plotter, along the X axis, and a carriage is moved perpendicular to the feed of the medium, along the Y axis. The carriage typically is adapted to receive a pen or similar marking device from a carousel or other magazine containing a plurality of pens, then to move the pen back and forth along the Y axis in order to develop the graphics (in conjunction with the movement of the paper along the X axis), and finally to return the pen to the carousel, either in the initial location or in a new location if the carousel has subsequently rotated. In order to facilitate retention of the pen by the carousel and by the carriage, the pen is typically disposed in a special adaptor or casing. It will be appreciated that other XY digital plotters utilize a stationary paper or chart and the pen carriage is capable of movement along both the X and Y axes.
The accuracy of the graphics produced by a digital plotter is limited by any inaccuracy of the pen tip itself--that is, the extent to which the position of the pen tip contacting the paper varies from the corresponding XY position of the carriage. Such inaccuracies may be caused by gross manufacturing defects or by misalignments resulting from trauma to the pen, the adaptor or the carriage. However, they may also be a result of even normal manufacturing tolerances of the pen (for example, such that the tip is not in perfect alignment with the central axis of the barrel), its adaptor or the carriage. Even the slightest positional errors in the position of the pen tip may become quite evident when a continuous line is formed using two or more pens. This may be required where the continuous plot is to consist of segments of different colors or simply where, in the midst of a plot, the current pen runs out of ink or otherwise becomes defective and requires replacement. Many other instances arise in which even a slight inexactitude in the disposition of the pen tip is unacceptable--for example, where different pens are to draw closely-spaced parallel lines.
Accordingly, it is an object of the present invention to provide an apparatus and method, for use in conjunction with a digital plotter incorporating controlled multiple marking devices to create displays on a medium, for determining the position of the marking devices along axes of movement.
Another object is to provide such apparatus and method for detecting and compensating for variation in the axial position of a current marking device along an axis relative to the axial position of a prior marking device.
A further object is to provide such apparatus and method which is functionally related to the position of the marking device by electrical signals from either electro/mechanical sensors or with one or more beams of radiation.
It is another object of the present invention to provide such apparatus and method which identifies the pen tip position detected by the sensors or beams and positions a successive pen tip to the position of a prior pen tip by first determining the location of the successive pen tip with the sensors or beams and correcting its position by moving the pen carriage, the medium, or both.